1. Field of the Invention
The present invention relates to methods and apparatus for separating, cooling and liquefying component gases from each other in a pressurized mixed gas stream. More particularly, the invention is directed to separation techniques that utilizes some of the components of the mixed gas stream that have already been separated to cool portions of the mixed gas stream that subsequently pass through the apparatus.
2. Description of the Prior Art
Individual purified gases, such as oxygen, nitrogen, helium, propane, butane, methane, and many other hydrocarbon gases, are used extensively throughout many different industries. Such gases, however, are typically not naturally found in their isolated or purified state. Rather, each individual gas must be separated or removed from mixtures of gases. For example, purified oxygen is typically obtained from the surrounding air which also includes nitrogen, carbon dioxide and many other trace elements. Similarly, hydrocarbon gases such as ethane, butane, propane, and methane are separated from natural gas which is produced from gas wells, landfills, city sewage digesters, coal mines etc.
In addition to separating or purifying the individual gases, it is often necessary to liquify gases. For example, liquified natural gas (LNG), which is primarily methane, is used extensively as an alternative fuel for operating automobiles and other machinery. The natural gas must be liquified or compressed since storing natural gas in an uncompressed vapor or gas state would require a storage tank of unreasonably immense proportions. Condensing or liquefying other gases is also desirable for more convenient storage and/or transportation.
The liquefaction of gases can be accomplished in a variety of different ways. The fundamental method is to compress the gas and then cool the compressed gas by passing it through a number of consecutively colder heat exchangers. A heat exchanger is simply an apparatus or process wherein the gas or fluid to be cooled is exposed to a colder environment which draws heat or energy from the gas or fluid, thereby cooling the gas. Once a gas reaches a sufficiently low temperature for a set pressure, the gas converts to a liquid.
The cold environment needed for each heat exchanger is generally produced by an independent refrigeration cycle. A refrigeration cycle, such as that used on a conventional refrigerator, utilizes a closed loop circuit having a compressor and an expansion valve. Flowing within the closed loop is a refrigerant such as Freon.RTM.. Initially, the refrigerant is compressed by the compressor which increases the temperature of the refrigerant. The compressed gas is then cooled. This is often accomplished by passing the gas through air or water cooled coils. As the compressed gas cools, it changes to a liquid. Next, the liquid passes through an expander valve which reduces the pressure on the liquid. This pressure drop produces an expansion of the liquid which may vaporize at least a portion thereof and which also significantly cools the now combined liquid and gas stream.
This cooled refrigerant stream now flows into the heat exchanger where it is exposed to the main gas stream desired to be cooled. In this environment, the refrigerant stream draws heat from the main stream, thereby simultaneously cooling the main stream and warming the refrigerant stream. As a result of the refrigerant being warmed, the remaining liquid is vaporized to a gas. This gas then returns to the compressor where the process is repeated.
By passing the main gas stream through consecutive heat exchangers having lower and lower temperatures, the main stream can eventually be cooled to a sufficiently low temperature that it converts to a liquid. The liquid is then stored in a pressurized tank.
Although the above process has been useful in obtaining liquefied gasses, it has several shortcomings. For example, as a result of the process using several discrete refrigeration cycles, each with its own compressor, the system is expensive to build, costly to run and maintain, and has an overall high complexity. A significant cost for any closed loop refrigeration system is the purchase and operation of the compressor. Not only does the compressor represent the process' largest capital expenditure, it also represents a major problem in the process system's flexibility. Once a compressor size is chosen, the process can only handle mass flow rates capable of being adequately compressed by the chosen compressor. In order to have wide flexibility in process flows, multiple compressors are then needed. These additional compressors also add to the cost and risk of equipment failure.
To make conventional systems cost effective to operate, such systems are typically built on a large scale. As a result, fewer facilities are built making it harder to get gas to the facility and to distribute liquefied gas from the facility. By their very nature, large facilities are required to store large quantities of liquified gas prior to transport. Storage of LNG can be problematic in that once the LNG begins to warm from the surrounding environment, the LNG begins to vaporize within the storage tank. To prevent pressure failure of the tank, some of the pressurized gas is permitted to vent. Such venting is not only an environmental concern but is also a waste of gas.
The steps for purification or separation of the different gases from a main mixed gas are often accomplished prior to the liquefaction process and can significantly add to the expense and complexity of the process. As a result, many productive gas wells having high concentrations of undesired gases or elements are often capped rather than processed.